1. Field of the Invention
The present invention relates to an error detection system having a programmable error sensitivity, and more particularly, to a digital audio data muting circuit which mutes a digital audio output of a receiver in a digital audio transmission system when a large number of errors per unit time are detected in the received digital audio signal.
2. Description of the Prior Art
Numerous processing systems are known for detecting and/or correcting errors in a received pulse code modulated signal in a communications system. Typically, when a small number of errors are detected in the received signal, means are provided for correcting the detected errors before the received signal is further processed. However, when such systems detect a large number of errors in the received signal which cannot be corrected by the available error correction circuits, the receiver output is typically disabled or, in the case of a digital audio transmission system, muted until such time as acceptable (i.e., relatively error free) data transmission is restored.
An example of a system of the type just described is disclosed by Horsten in U.S. Pat. No. 4,864,573. Horsten therein describes a muting circuit for a compact disc digital audio system having error correction means for correcting errors in data words forming the pulse code modulated signal. In that system, when the error correction means cannot correct all of the errors in the pulse code modulated signal, error flags are produced on an output and applied to a control unit which derives from these flags a control signal for switching on a muting unit when a predetermined number of flags has been received. As a result, the digital audio output of the compact disc system is muted when a large number of errors (flags) are detected.
The muting unit in the Horsten apparatus is not switched on until there are definitely errors in the samples, and the muting unit is not switched off until there are only a few errors left and the signal is again of more or less hi-fi quality. Unfortunately, Horsten does not distinguish different types of transmission errors, and accordingly, the digital audio output is not muted until the requisite number of flags are detected in a predetermined time interval. As a result, the digital audio output received by the listener may be perceptibly degraded before the output is completely muted. A more robust muting system is desired which further discriminates amongst different types of transmission errors so that the data muting may occur sooner when more serious errors are detected, thereby preventing the listener from perceiving a degradation of the digital audio signal before the digital audio receiver is muted.
Other techniques are known for muting digital audio outputs when errors in the received signal are detected. For Example, Kouyama in U.S. Pat. No. 4,593,392 mutes the output of a digital tape recording/reproducing apparatus when an error is detected in the received signal. Complementary gain control circuits are used by Kouyama to gradually fade in a newly selected signal (which may be, e.g., a previous value of the received signal or a muting signal) while gradually fading out the previously selected signal, allegedly eliminating undesirable clicking noises.
Similar systems for muting digital audio outputs when errors are detected in the received data include U.S. Pat. No. 4,433,415 to Kojima and U.S. Pat. No. 4,962,494 to Kimura. Kojima discloses a system in which the digital audio output is muted when an input memory buffer overflows, while Kimura discloses a volume control circuit in which the output is muted "according to necessity."
Muting has also been used in speech transmission systems when data errors are detected in the received signals. Such techniques are described by way of example by Coombes et al. in U.S. Pat. No. 4,312,070; McNair in U.S. Pat. No. 4,608,455; Dal Degan et al. in U.S. Pat. No. 4,688,224; and Rasky in U.S. Pat. No. 4,802,171. However, as with the digital audio data processing systems, such speech transmission systems do not provide a robust muting system which discriminates amongst different types of transmission errors so that the data muting may occur sooner when more serious errors are detected so as to prevent the listener from perceiving a degradation of the digital audio signal before or after the output is muted.
A digital audio data transmission system has been developed by the assignees of the present invention which permits the transmission of digital audio signals over a satellite transmission network and then distribution of the digital audio signals over local cable television channels for reception by a subscriber. Such a system is described, for example, in U.S. patent application Ser. No. 07/618,744, filed Nov. 27, 1990. As described therein, digital audio signals from a plurality of compact disc players are encoded and transmitted over the satellite transmission network and retransmitted over a local cable distribution system for reception by a digital music terminal in the homes of the respective subscribers. Such a system provides high quality digital audio signals to the subscribers without the interruption or noise which is prevalent in common AM and FM audio signal transmission systems.
However, under certain circumstances, such as an interruption in the satellite transmission, it is desirable that the digital audio output be muted to prevent perceptible degradations of the received audio signal. Although error detection and correction systems are provided in this system to correct or cover up less serious errors so that the listener does not perceive the data errors, under certain circumstances more errors are detected than can be corrected by the error correction circuitry. Under these conditions, it is desired that the digital audio output be muted until the signal quality returns to an acceptable level. Since not all data transmission errors cause the same degree of perceptible degradation in the received digital audio signal, it is also desired that different type of transmission errors be discriminated so that the digital audio output may be muted sooner when errors more perceptible to the listener are detected. The present has been designed to meet these needs.